I. Field of the Invention
The present invention relates to data flow through a frame relay network and, more particularly, to a system and method for controlling the data flow from a LAN (local area network) station through a frame relay network when the frame relay network becomes congested.
II. Background and Prior Art
As users have moved along the path toward decentralized processing, local connectivity problems have been solved by attaching network elements--workstations, minicomputers, and microcomputers--via local area networks (LANs). Further, as users wanted to have global communications that make any device anywhere appear as if it were attached to the local LAN, LANs spread across the globe have become internetworked across wide area networks (WANs). Because LAN interconnect data is bursty and unpredictable, "fast packet" services, which provide bandwidth on demand (such as frame relay), are the networking technologies of choice for interconnecting LANs.
Frame relay is a multiplexed data networking service supporting connectivity between internetworking units (IWUs), such as bridges and routers, and between IWUs and carrier networking equipment. IWUs which are normally located at the customer premises may be called customer premises equipment (CPE). In frame relay, which can be considered the successor to the CCITT X.25 packet standards, error correction and flow control are handled at network end points, i.e., at the IWUs. Frame relay accelerates the process of routing packets through a series of switches to a remote location by eliminating the need for each switch to cheek each packet it receives for errors before relaying it to the next switch. Instead, only the IWUs at the network end points cheek for errors. This error treatment increases performance and reduces bandwidth requirements, which in turn can reduce communications costs and decrease the number of packet handling devices needed in the network.
Furthermore, flow control is conducted by the IWUs at the end points of the network only--no flow control is presently conducted between the user and the network end point. Thus, the IWU connecting with the frame relay network is charged with controlling the flow of data frames into the network, i.e., slowing down or speeding up the data traffic into the network based upon the network's present load.
The frame relay technologies are defined as having to provide the IWU with a congestion notification so that the IWU can assist in clearing the network by not forwarding any additional data frames until the network is adequately clear for transmission. These congestion notification messages are denoted as FECN (forward explicit congestion notification) and BECN (backward explicit congestion notification) and indicate that the network is congested. These indicators, as well as the rate control concept based upon CIR (committed information rate), are used to protect the frame relay network from congestion. The IWU receiving either of these indicators (FECN or BECN) responds accordingly, as will be discussed below.
With regard to the LAN data traffic, in a normal LAN environment, i.e., in an environment where there is no LAN interconnection, stations connected to the LAN perform certain network control functions, such as error correction and flow control. With regard to flow control, LAN stations conforming to the IEEE 802.2 recommendation will provide a logical link connection (LLC) protocol element which implements a dynamic window flow control mechanism. This mechanism when activated will reduce the window size through which data may pass to a fraction of its original value, and in accordance with the success of subsequent transmissions, the window size will automatically increase, until it returns to its original value.
Due to the use of the IWU at the boundary between the frame relay network and the LAN, however, the frame relay network is transparent to the LAN station and, thus, the LAN station cannot perform any flow control function. The LAN station has no way of knowing that the frame relay network is congested as it does not know that the frame relay network is even there. Thus, the LAN station of the prior art must depend upon the IWU at the network boundary to receive all of the data destined to cross the frame relay network and to control its flow when the network becomes congested.
FIG. 1 shows an example of a network 10 having four local area networks (LAN1 12, LAN2 14, LAN3 16 and LAN4 18) interconnected through a frame relay network 15. Station A1 22 is connected directly to LAN1 12 while station A2 24 is connected directly to LAN4 18. Bridges B1 26 and B2 28 perform bridging functions to interconnect LAN1 12 and LAN2 14 (B1 26) and to interconnect LAN3 16 and LAN4 18 (B2 28). Routers R1 30 and R2 32 perform the routing functions to interconnect LAN2 14 and LAN3 16 to the frame relay network 15, respectively. Routers R1 30 and R2 32 represent the IWUs at the frame relay network end points as discussed above.
In present systems, when the frame relay network 15 becomes congested, it sends to routers R1 30 and R2 32 an indication to slow the flow of data into the network. These are the B ECN and FECN indicators discussed above. Upon receiving either of these indicators, router R1 30 (R2 32) will store the frames of data that it receives from LANs 1 and 2 (LANs 3 and 4) until the data can be safely transmitted over the network 15. In the meantime, however, stations connected to each LAN continue to forward frames of data to router R1 30 (and router R2 32) as if the network were clear. Router R1 30 (R2 32) merely stores this data in a queue in its buffer storage area until there is an indication from the network that a higher volume of data may pass.
For instance, when a frame is sent from a user (which is a router in the above example) to its target through a frame relay network, if the network experiences congestion in the same direction as the frame to going, FECN will be activated. If the congestion is in the opposite direction, BECN will be activated. The frame relay network expects users to slow data flow when BECN or FECN is activated, the manner in which the traffic is slowed is at the user's discretion. Many users, however, do not take appropriate actions and continue to pump frames into the network which results in frames being discarded by the network.
In the cases where a router takes appropriate action and slows the data traffic to the congested network, there is a problem in the case where the router is receiving too much data for transmission across the frame relay network. If the network is congested, the router stems the data flow by storing the data that it receives in internal buffers but, because it is receiving so much data from the LAN stations, it may run out of buffer storage space for the incoming data. Because it has no mechanism for indicating to the attached stations to slow or stop the data flow, this data will be discarded and lost.
Even where the router or other CPE has sufficient storage space for storing all of the outgoing data frames, the router cannot retrieve the stored data frames quickly enough for transmission onto the network after the network is clear as this requires much processing effort. Thus, the router (or other IWU) becomes the bottleneck to the LAN data traffic. This problem is especially common when the router receives high volume, bursty data traffic from the LAN stations for transmission over the frame relay network.
Thus, it can be seen that with present systems, there is no mechanism for efficiently interconnecting local area networks (LANs) through a frame relay network. Present systems do not allow workstations connected to the LANs to provide flow control for data that the workstations are transmitting over the frame relay network when the network becomes congested. Instead, the network end points are solely responsible for this task which ultimately results in the end points themselves becoming the bottlenecks to the network traffic as their buffer storage overflows and data is lost.